It is well-known that if a gas stream is caused to flow upward through a bed of solid particles at a sufficient rate of flow, the particles in the bed move freely instead of resting upon each other and the bed behaves much as a liquid. These fluidized solid particles exhibit buoyancy of floating objects, surface waves, and other properties normally associated with liquids. High rate of mixing and heat transfer is provided by such conventional beds, making them applicable to various drying, roasting, chemical and petroleum processes, as is also well known. A further advantage in the use of a fluidized bed in said processes is that the continuous addition and removal of the solids which make up the fluidized bed provides for convenient means for removal of fines formed by the breakdown of the solids and spent catalyst particles when said fluidized solids are used in a catalytic manner.
One serious disadvantage of gas-fluidized solids has been noted in the art, this being that as the velocity of the gas is increased to above a minimum value, bubbles are formed in the bed. A bubbling fluidized bed has regions of low solid density comprising gas pockets or voids, that are referred to as gas bubbles. The formation of bubbles leads to bypassing, slugging and channeling, which results in the loss of the intimate contact between the fluid and the solids that is expected in a fluidized bed process.
Various methods have been tried in the prior art to stabilize fluidized beds by preventing the bubbling or "boiling" phenomenon, including the use of corona discharges (U.S. Pat. No. 3,304,249) and applied magnetic fields (U.S. Pat. Nos. 3,439,899 and 3,440,731). A more recent solution to the problem is that provided by R. E. Rosensweig in copending U.S. application Ser. No. 610,071, filed Sept. 3, 1975, to which Belgium Pat. No. 834,384 corresponds. The present invention provides an improvement in the invention of Rosensweig when it is to be applied to processes that involve heat transfer either by release of heat or by the absorption of heat.
Briefly, in the Rosensweig invention there are included, in the bed of solid particulate material making up the fluidized bed, a plurality of separate, discrete magnetizable particles, and the bed is fluidized by a stream of gas flowing upward through the bed in the usual manner. There is applied to the fluidized bed a substantially uniform magnetic field which is oriented with a substantial vertical component. The strength of the magnetic field and its deviation from a vertical orientation are maintained so as to prevent formation of bubbles in the fluidized bed for the existing gas flow rate and particulate solids makeup of the bed. This enables use of gas throughput rates that are as much as 10 to 20 times as great as the flow rate of the gas at incipient fluidization in the absence of the applied magnetic field, concomitant with the absence of bubbles. Such a magnetically stabilized medium has the appearance of an expanded fixed bed; there is no gross solids circulation and very little or no gas bypassing. A bed of the magnetically stabilized medium shares many qualities of the normal fluidized bed; pressure drop is effectively equal to the weight of the bed and independent of gas flow rate or of particle size; the media will flow, permitting continuous solids throughput. Beds of the magnetically stabilized media also share some of the qualities of a fixed bed; countercurrent contacting can be readily attained; gas bypassing is small or absent, making it possible to achieve high conversions; and attrition is minimal.
Although magnetically stabilized fluid beds have a number of advantages over both fixed beds and the conventional fluidized beds, including low rates of particle attrition and high fluid flow rates at low pressure drops, they do have one inherent disadvantage, in that they have a very limited ability to permit the transfer of heat both between the fluidized bed and the walls that confine it, and within the fluidized bed to and from objects immersed therein to remove heat from or to add heat to the fluidized mass. The limited ability of such beds to transfer such heat is of little or no consequence in those cases where the beds are being used in processes that do not involve a large release or absorption of heat. In the great majority of applications of fluidized beds, however, chemical reactions and/or physical changes occur that are accompanied by thermal effects, as for example in evaporation or drying or in exothermal or endothermal reactions. Substantial increases in temperature within such fluidized beds can be undesirable for many reasons. For example, they can cause thermal degradation of fluids passing through the bed, changes in the selectivity of chemical reactions taking place in the bed, and thermal degradation of the particles in the bed, thereby shortening their useful lifetime. Also, when temperatures exceed the Curie temperature of the magnetic particles in the bed they will lose their magnetic properties and thus prevent stabilization of the bed with a magnetic field. Additionally, hot regions in the bed can cause gas expansion to the extent that the gas velocity in those regions exceeds the maximum velocity at which magnetic bed stabilization can be achieved for the strength of the magnetic field being employed.
Similarly, in those cases where heat is being absorbed rather than released during the process occuring in the magnetically stabilized fluidized bed, substantial declines in temperature in localized areas can also lead to undesirable conditions, including reduction in the rate of chemical reaction, reduction in the rate of physical change, condensation of a normally gaseous component of a fluid passing through the bed, and changes in selectivity of chemical reactions, whether catalytic or non-catalytic.
In addition to the above-noted problems, the non-isothermal nature of magnetically stabilized fluidized beds makes difficult the prediction of the behavior of such beds, in respect to both physical and chemical properties.
The present invention provides a method to improve thermal characteristics of magnetically stabilized fluidized beds, and overcomes the problem of thermal gradients in such beds, without sacrificing the advantages of such beds.
This novel method for control of thermal characteristics of magnetically stabilized fluidized beds consists of periodically removing the stabilizing magnetic field from the fluidized bed and then reapplying said field. The relative lengths of time in the "field on" and "field off" modes is determined from the characteristics of the bed and the nature of the processing that is occurring in the bed. The more exothermic or endothermic the process is, the less must be the ratio of the length of time the field is on to the length of time the field is off. This ratio may range from as small as 4 to 1 to as great as 4000 to 1, but the preferred range is from 8 to 1 to 400 to 1. The physical configuration of the magnetically stabilized fluidized bed (i.e., particle size and type, fluid velocity and physical properties, bed size and geometry and magnetic field strength, orientation and uniformity) determines the absolute period of "field off" mode of operation. Each case must be determined individually, using the criterion that the "field off" mode must end before the "boiling" or "bubbling" bed typical of unstabilized fluidized bed operation becomes evident.
Operation is cyclical, i.e., "field on" and "field off" modes follow each other in regular succession. During the "field on" mode, the magnetically stabilized fluidized bed begins to develop temperature gradients. Before these gradients become significant, the "field off" mode begins, during which the particles in the bed mix sufficiently to scramble any temperature gradients which appeared during the "field on" mode. The "field on" mode then resumes, whereupon temperature gradients may again begin to develop, only to again be scrambled by the "field off" mode; and so forth.
Careful selection of the length and frequency of the "field on" and "field off" modes is necessary for proper operation of this method of obtaining a magnetically stabilized fluidized bed that is essentially free of undesirable thermal gradients. The length and frequency to give the desired operation may be experimentally determined and the bed then run at that fixed condition of repetitive "field on" and "field off" modes. Alternatively, the "field on" mode may be maintained until thermal sensors detect sufficient departure from desired absolute temperature levels or acceptable temperature gradients, whereupon the "field off" mode follows for the desired period, followed by reversion to "field on" mode; and so forth. In this case, the "field off" to "field on" ratio is not determined in advance and held fixed; rather the ratio is determined by the behavior of the process at any particular time.
In general, the duration of the "field off" mode will not exceed twice the residence time of the fluidizing gas in the bed; most preferably the time off will be equal to about the residence time of the gas. Residence time in most fluidized beds will be less than 20 seconds, more usually 4 to 10 seconds. In the example which follows, wherein the area of the bed was about 20 cm.sup.2 and the gas flow rate was about 73.3 ml. per second, the gas residence time was about 4 seconds and the duration of the "field off" mode was 2 seconds. In general, the minimum time off should be determined by the desired level of mixing of the solids and the practical limits afforded by a control system for short times of the "field off" mode. The duration of the "field on" mode will be determined by the level of temperature increase or decrease or of concentrates profile desired or considered permissible in the particular process that is involved. It is to be remembered that the key factor is the obtaining of solids mixing without bubbling in the bed.